1. Field of Invention
The present invention relates to furnaces designed to establish a temperature gradient in the work volume and particularly to such furnaces capable of varying the temperature gradient under precise control.
2. Description of the Prior Art
Temperature gradient furnaces are used in a number of production processes. One example is in the "growing" of large monocrystals. A steep and well-controlled temperature gradient is necessary. The simplest and most common approaches have been to provide a static temperature gradient within the furnace and then either: (a) move the work through the gradient holding the furnace stationary or (b) move the furnace while holding the work stationary. In U.S. Pat. No. 4,086,424 issued to the present inventor, a furnace and method are described in which both furnace and work remain stationary while the temperature gradient within the furnace is made to move. In any of these furnaces, it is difficult to obtain good precision control over the temperature gradient or gradients unless good control is established over heat dissipation. One problem is the result of heat flow radially to the axis of the temperature gradient. Such heat flow produces undesired temperature variations within planes perpendicular to the axis of the gradient. Another problem is related to loading of the heating elements. Unless a heating element is continually loaded, its response to control can be slow and erratic. In order to obtain steep uniform gradients, these problems have to be overcome.